One approach to the preparation of thin wafer structures of materials for optical, electronic, and other uses is to deposit the material of the wafer structure onto a substrate. The material is furnished in the liquid phase or the gaseous phase and then transported to the substrate, where it deposits as a solid. The wafer structure gradually builds up its thickness on the substrate in a layer-by-layer manner, so that it is of good purity and high crystal perfection.
The substrate is selected to be compatible with the nature of the wafer structure and with the deposition technique. The substrate must allow the growth of the desired crystal structure of the wafer structure, must be chemically compatible with the wafer structure so that the wafer structure is not contaminated during growth, and must be stable in the conditions of deposition. Additionally, the substrate should not by its thermophysical properties introduce imperfections or damage into the wafer structure as it is deposited, further processed in contact with the substrate, and cooled to room temperature.
In an example, a wafer structure of HgCdTe (mercury-cadmium-telluride) used in an infrared sensor is fabricated by deposition onto a CdZnTe (cadmium-zinc-telluride) substrate using molecular beam epitaxy. However, the physical size of CdZnTe substrates is limited by the available production techniques, so that the lateral area of the HgCdTe wafer structure is limited, under currently available technology, to about 30 square centimeters. It is desired to have larger HgCdTe wafer structures, to allow the production of larger-size focal-plane-array (FPA) infrared detectors.
It has been proposed to use silicon as the substrate to grow larger HgCdTe wafer structures, as large silicon substrates may be readily prepared. However, the HgCdTe wafer structures produced by deposition onto silicon substrates contain high densities of defects that render the HgCdTe wafer structures unusable for many detector applications.
There is accordingly a need for an improved approach to the fabrication of HgCdTe wafer structures, as well as other types of deposited crystals, that is satisfactory for small-area wafer structures, but is also operable for the preparation of wafer structures of larger surface areas. The present invention fulfills this need, and further provides related advantages.